The purpose of the present study is to assess whether or not patients on antidepressant medications have changes in mood following acute tryptophan depletion and to relate those changes to certain clinical variables including type of antidepressant medication, duration of remission of depressive symptoms, and the history of recurrence of depression following cessation of antidepressant treatment. This study is investigating the role of the brain serotonin system in depression by measuring the effects on mood of a drink that is low in the amino acid tryptophan. Tryprtophan occurs naturally in a normal diet and is the only amino acid which is naturally converted into serotonin. Rapid reduction in blood tryptophan levels has been found to lead changes in mood in some patients. Approximately 30 subjects are expected to enroll in the present study. Following a thorough medical and physical evaluation, outpatients will be scheduled for two tryptophan depletion tests. Tryptophan depletion tests will consist of two sessions approximately one week apart. Each test session will include one day of 160 mg/day of tryptophn diet followed the next morning by an amino acid drink with 25 amino acid capsules. During one test session plasma tryptophan levels will be lowered by 90% (acute tryptophan depletion test) and during the other session plasma tryptophan levels will be lowered only 10-20% (control test). At specifed intervals throughout the test sessions, blood will be drawn for plasma sampling and patients will be given psychological rating scales. To date, 5 of the expected 30 patients have been enrolled.